1. Field of the Invention
This invention relates to a method for the manufacture of a magnet. More particularly, this invention relates to improvements in and concerning a method for the manufacture of a magnet by the magnetization of an annular material prepared for magnetization.
2. Description of the Prior Art
Generally, in flat motors and other similar electric devices, an annular magnetized shaped article of magnetic material (such as barium ferrite, SmCo.sub.5 or Alnico) of a relatively small wall thickness, which is called a ring magnet, is used.
The annular magnet of this class is magnetized, as illustrated in FIG. 1, for example, by setting an annular material 12 for magnetization in place on a base 14 as fixed to a yoke 10, applying a magnetizing member 16 hung down from above on the surface of the material 12, and energizing a coil 18 wound on the magnetizing member 16. This magnetizing member 16 comprises a core member of magnetic material (actually an iron core) 18 and one magnetizing coil 20 wound up in alternately opposite directions on the core member, particularly the projected parts 18a and 18b thereof.
When the magnetization is effected only on one side of the annular material as described above, however, there is produced a magnet M.sub.1 having narrow intermediate zones 22 interposed between adjacent S poles and N poles as illustrated in FIG. 2, partly because the projected parts 18a and 18b of the core member 18 are separated from each other only by a small distance. If the magnet M.sub.1 of this nature is incorporated in a rotary machine such as the flat motor, there is the possibility that the device will generate electromagnetic vibration when it is set rotating. By the term "electromagnetic vibration" as used herein is meant the vibration which occurs because the magnetic attraction manifested by adjacent S and N poles is altered when a live armature passes a point of magnetic flux shift from an S pole to an N pole, for example. It forms a possible cause for motor noise and degradation of motor performance.
As a remedial measure, the practice of carrying out magnetization while fixing the distance, d, between the projected parts 18a and 18b of the magnetizing member 16 at a prescribed length as illustrated in FIG. 3 has found popular acceptance. Despite this contrivance, the magnetic flux induces interference between adjacent poles because the magnetic flux, in the presence of electric current, flows from the S pole of the annular material 12 through the yoke 10 to the N pole of the annular material 12, making a U-turn flow inside the annular material 12. As the result, the parts which should form neutral zones 24 are actually magnetized as indicated by hatching as illustrated in FIG. 4. Thus, with the ring magnet M.sub.1, it is difficult to obtain parts ideally devoid of magnetic flux. This imperfect absence of magnetic flux in these parts may be blamed for the motor noise and the degradation of motor performance.
Recently, therefore, a split magnet M.sub.2 which has intervening neutral zones 24 formed positively as illustrated in FIG. 5 has come to find acceptance. Since the split magnet M.sub.2 necessitates attachment of a multiplicity of magnets 12 independently to the yoke 10, it entails operational incoveniences from the standpoint of manufacture and inevitably proves expensive.
An object of this invention is to provide a novel method for the manufacture of a magnet.
Another object of this invention is to provide an improved method for the manufacture of a magnet by the magnetization of an annular material.
Yet another object of this invention is to provide a method for the manufacture of a magnet, which ensures formation of sharply defined neutral zones without any sacrifice of the ease of fabrication enjoyed by the ring magnet.